Magnets and migrations
Navigating without GPS
It was mid-September before I got around to taking in their feeder. By that time the pair of Rufous Hummingbirds, and the family they raised in our small corner of Alpine Meadows were probably already sipping nectar from tropical blooms on the high Mexican plateau. When some of them return next April, as they have for many years past, they will have completed a round trip of nearly 6,000 kilometres.
To duplicate this marvel of navigation a human traveller would require at least a clock, a sextant, and a compass. We know that birds orient on the sun, moon, and stars and, in conjunction with their biological clocks, use these visual clues for navigation. But we also know that many long-distance migrants find their way when the night sky is covered, and when crossing open ocean as the Ruby-throated Hummingbird does in its 800 km non-stop flight across the Gulf of Mexico. It seems that birds do indeed have a built in compass that is independent of visual clues.
Experiments indicate that many migrating species, ranging from birds to turtles, use the earth's magnetic field for navigation. Homing pigeons displaced in closed aluminum boxes have no problem retracing their outbound route back home, even when their vision is restricted by frosted contact lenses. Yet pigeons displaced in iron boxes, which mask the magnetic field, are disoriented and unable to return.
No one knows who invented the magnetic compass but we have a pretty good idea what the prototype looked like: a chunk of magnetite suspended on a string or floated on a chip of wood. By the 11th century both of these early models were being used by mariners, who were at last able to stay on course when the sky was covered. Once known as "lodestone", meaning "way stone", magnetite is a common and widespread mineral (Fe3O4) which has a strong natural magnetic polarity a well-defined N and S pole which allows it to serve as a compass needle.
In 1979 James Gould and Charles Walcott found what could be the biological compass, a tiny crystal of magnetite, the same mineral used in early man-made compasses, in the head of a passenger pigeon. Since then new research has shown that biogenic magnetite embedded in nerve fibres is present in a host of other migratory species. In fact the ability to respond to the earth's magnetic field is such a common phenomenon that in his book The Mystery of Migration Robin Baker speculates that future investigations may concentrate on looking for an animal that cannot detect it.
But the mystery of migration remains. Who knows what will guide my pair of Rufous Hummers back from Mexico the sun moon and stars, the sights sounds and smells along the route, or the complex pattern of magnetic variation. Probably all of these things, stored in their tiny brains like the waypoints in my GPS leading the way back home to Alpine Meadows. I'll have the feeder out and waiting.
Whistler Naturalists Annual General Meeting Takes place on Wednesday, Nov. 10 from 6 to 7 p.m. at Millennium Place. All current members and anyone interested in getting more involved is encouraged to join us for our 6th AGM. The AGM will include elections to the board; new board members welcome. For information contact Tracy Howlett: 604-935-9745.
Whistler Naturalists Speaker Series Whistlers Old Trees: A Story of Fires, Floods, Pathology, and Patience with Bob Brett. Bob will present his study of over 500 trees from all over Whistler. Hes used tree rings to determine ages and how individual forest stands began. Event takes place on Wednesday, Nov. 10 at 7:30 p.m. at Millennium Place, doors open at 7 p.m. Admission is $5 with a 2004 Whistler Naturalists membership or $7 for non-members (children under 12 free). Tickets and memberships available at the door.
Monthly Bird Walk The next bird walk will take place Saturday, Nov. 6th and will start at the later fall/winter time of 8 a.m. Join Whistler experts in the monthly update of our feathered locals and migrants. For details, contact Michael Thompson at 604-932-5010.